Attenuation of morphine antinociceptive tolerance by CP-55940, LY235959, or CP-55940/LY235959 mixtures. Abscissae, total drug dose (mg/kg) of CP-55940, LY235959, or CP-55940 and LY235959 administered in combination with 100 mg/kg morphine during the chronic regimen. Ordinate, relative potency of morphine as assessed before and after chronic drug administration. Data point above “M” represents the relative potency after chronic administration of 100 mg/kg morphine alone. Each data point represents the mean (± S.E.M.) from 7-8 mice.